We designed and evaluated a unipolar diffusion charger. This charger consisted of a corona discharge, mixing, and ion trap zones. For the performance evaluation of the diffusion charger, sodium chloride (NaCl) particles smaller than 0.1 mu m in diameter, and dioctyl sebacate (DOS) particles of 0.1-0.7 mu m, were used. The diffusion charger was evaluated with two parameters. One parameter was particle loss and the other parameter was charging characteristics which were the product of penetration and average number of charges (P . n). The parameters were measured by varying values of the voltage applied to the diffusion charger. The total particle losses were below 15%. The number of charges was almost linear with respect to the particle diameter. In order to reduce the time required to obtain the desired particle size distribution in the laboratory experiments, we estimated the geometric mean diameter and the log-normal size distribution of the generated submicron particles by using a diffusion charger as well as a condensation particle counter (CPC, TSI model 3025A) and an aerosol electrometer (TSI model 3068A). The estimated size distribution can be used for real-time measurements after calibration. (c) 2007 Elsevier Ltd. All rights reserved.